TALIN is a high-molecular-weight cytoskeletal protein concentrated at regions of cell–substratum contact and, in lymphocytes, at cell–cell contacts. Talin is a ubiquitous cytosolic protein that is found in high concentrations in focal adhesions. It is capable of linking integrins to the actin cytoskeleton either directly or indirectly by interacting with vinculin and alpha-actinin. Integrin receptors are involved in the attachment of adherent cells to extracellular matrices and of lymphocytes to other cells. In these situations, talin codistributes with concentrations of integrins in the cell surface membrane . Furthermore, in vitro binding studies suggest that integrins bind to talin, although with low affinity. Talin also binds with high affinity to vinculin, another cytoskeletal protein concentrated at points of cell adhesion. Finally, talin is a substrate for the Ca2+-activated protease, calpain II, which is also concentrated at points of cell-substratum contact.
Talin consists of a large C-terminal rod domain that contains bundles of α-alpha helices and an N-terminal FERM (band 4.1, ezrin, radixin, and moesin) domain with three subdomains: F1, F2, and F3. The F3 subdomain of the FERM domain contains the highest affinity integrin-binding site for integrin β tails and is sufficient to activate integrins.
Talin Activates Integrin αIIbβ3
A structure-function analysis reported recently provides a cogent structural model (see top right) to explain talin-dependent integrin activation in three steps:
♦ (A) The talin F3 domain (surface representation; colored by charge), freed from its autoinhibitory interactions in the full-length protein, becomes available for binding to the integrin.
♦ (B) F3 engages the membrane-distal part of the β3-integrin tail (in red), which becomes ordered, but the α-β integrin interactions that hold the integrin in the low-affinity conformation remain intact.
♦ (C) In a subsequent step, F3 engages the membrane-proximal portion of the β3 tail while maintaining its membrane-distal interactions.
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